The present invention relates to a semiconductor device having a protection circuit to prevent the device from destruction due to excess voltage induced by electric surge or pulse noise. It particularly relates to a protection circuit having an ability to withstand excess input voltage, while still providing the inner elements of the device to be protected with a suitably low protection voltage.
In a semiconductor device, to improve its realiability, a protection circuit having protective elements such as semiconductor diodes is formed together with the inner elements such as MOS FETs (metal oxide semiconductor type field effect transistors) on the semiconductor substrate. The protection circuit prevents inner elements from being destroyed due to high voltage derived from an external noise pulse, contact with a charged human body, etc. Since the ICs (integrated circuits), such as memory ICs, have made remarkable progress in integration and high speed information processing ability, the structure of the elements formed in the IC becomes fine and delicate.
Particularly, an insulating layer under the gate electrode of a MOS FET formed in the device is so thin that it is often broken down by the applied electric field from a relatively small amount of electric charge supplied by an exterior source, such as by a charged human body.
In addition, the size and depth of the impurity regions of the inner elements have become smaller with the progress mentioned above. This tends to further reduce the capability of the protective elements formed in the device to withstand the excess voltage.
An impurity region of an element of a semiconductor device is usually formed by diffusing an impurity material (dopant) into a specified region in a substrate, which is referred to as a "diffusion region" in the following. The protection circuit elements also have diffusion regions, which are formed in the same diffusion process for forming the inner elements, so that the depth of the diffusion regions for the protection elements is the same as the depth of the diffusion regions of the inner elements.
Generally, in reverse-biased semiconductor rectifiers, such as diodes, diode-connected transistors, etc., the breakdown voltage decreases as the depth of the diffusion region becomes shallower. The breakdown voltage is defined as a voltage at which a reverse-biased p-n junction ceases to have sufficient impedance. Usually, the impurity region formed by a thermal diffusion process has a side wall with a radius of curvature r.sub.i (when the side wall is seen in cross-section). This radius r.sub.i is almost equal to the diffusion depth. So the deeper diffusion region has a larger radius of curvature along the boundary of the region. A large radius reduces the concentration of the electric field, resulting in an increased breakdown voltage of the element.
When the reverse current at the breakdown voltage exceeds a critical value, the junction of the diode will be melted and damaged locally at a portion where the current concentrates. The energy initiating the damage of the junction is defined herein as the "burn-out energy" of the diode. A large radius of curvature r.sub.i of the side wall of the diffusion region also causes the concentration of the diode current to decrease, resulting in a relatively uniform distribution of heat generation at the junction. Therefore the burn-out energy increases with the depth of the diffusion region.
The term "surge capacity" is defined herein as the ability of a diode to withstand the voltage or current transients in excess of its normal rating. The surge capacity is influenced by many factors, but the burn-out energy plays a main role. Thus, increasing the burn-out energy of a protective element is regarded as equivalent to improving its surge capacity.
From this point of view, a semiconductor device having protection elements with a deeper diffusion region than that of the inner elements was proposed by T. Yamanaka in TOKUGAN SHO55-11864 filed in the Japanese Patent Office in 1980. Therein, the elements in the protection circuit of the device are expected to have a higher burn-out energy and an increased surge capacity. But, as a result, the breakdown voltage itself goes up, as described above. This is not desirable for protecting the inner elements of the device, since a higher voltage is provided to the inner elements as the breakdown voltage of the protective element goes up.
As stated before, the depth of the diffusion region of the protective elements is required to be thinner to provide the lower protection voltage. This means that there are conflicting requirements for a protective element of the protection circuit, that is, between realizing a higher surge capacity while at the same time reducing or maintaining the protection voltage. Therefore, the protective elements of the above prior art are becoming increasingly unsatisfactory for protection circuits.
Semiconductor elements for large output are usually provided and, generally, the output terminals are connected to the diffusion regions of such elements, so that the elements themselves act as protective diodes. Therefore no special protection circuits have been arranged for the output circuits. In addition, in an output circuit, a diffusion region for a resistor is generally not provided, in order to avoid reduction of the output level. But in a circuit with high output voltage, such as a driving circuit for fluorescent display tubes, some protecting means for protecting the inner elements becomes necessary.
Recently, an input/output terminal for both input and output has become frequently used in a one-chip microcomputer or other IC device. In such a device, a protection circuit for the output circuit is also necessary, in order to assure reliability of the IC device.